There is a pressing need in industry for a monitoring system capable of identifying and quantifying trace quantities of hazardous gasses escaping into the air. The monitors, located in the vicinity of processing plants and storage facilities, provide early warning of impending danger to plant personnel and to the public, and they enable corrective action to be taken in time to avoid disaster. An ideal monitoring system is capable of operating unattended continuously for extended periods of time without the need for frequent maintenance or calibration. Monitoring systems in present use electrochemical cells and other devices as the sensing element. Such systems are inadequate in some applications because they lack specificity, require frequent maintenance for calibration and replenishment of electrolyte, and many are limited to operation at ambient temperatures above 0.degree. C. (due to freezing of the electrolyte).
The Ion Mobility Spectrometer (IMS) is an accepted analytical tool capable of identifying and quantifying trace amounts of a substance in a sample. Basically, an IMS comprises an analyzer cell, means for ionizing samples of an analyte admitted to the cell and means for determining the times required for the ions of the various substances present in the cell to traverse a specific length of the cell under the influence of an electric field and against the force of a stream of drift gas flowing through the cell in a direction opposite to that of the electric field. A representative analyzer cell is disclosed in U.S. Pat. No. 4,390,784 issued to Browning et al. A stream of purified air or other gas may be used as a carrier gas to introduce the analyte sample into the cell, and a stream of purified air or other gas may also be used as the drift gas. Both the carrier gas and the drift gas are therefore readily available at an installation site in unlimited quantities and no maintenance is required of the sensor other than the occasional replacement of filters and membranes for purifying the carrier and drift gasses, radiation wipe tests and calibration. An IMS therefore appears to be the ideal sensor for use in a monitoring system.
However, it has been found that an IMS operated in a conventional manner, using air as the carrier and drift gasses, may lack the specificity necessary to detect many of the acid gasses of interest, such as hydrogen fluoride, hydrogen chloride, nitrogen dioxide, and others. The reason for such lack of specificity is that the ion peak characteristic of pure air alone and the ion peak characteristic of the analyte gas in air both arrive at the ion detector of the IMS at virtually the same times. The pure air ion peak disrupts the analyte peak, especially when the analyte is an acid gas. Hence, it becomes very difficult to distinguish the amplitude of the ion current due to the acid gas from the ion current due to pure air at a given arrival time.
In application Ser. No. 534,701 now U.S. Pat. No. 5,032,221, a monitor having improved specificity for acid gasses was described. The disclosed acid gas monitor makes use of a dopant selected from the group of substituted phenols. The dopant improves the ability of the gas monitor to detect the general presence of acid gasses such as hydrogen flouride, hydrogen chloride, chlorine, nitrogen dioxide, sulfur dioxide, carbonyl sulfide, and numerous others. This ability to detect the presence of acid gas proves advantageous in many situations. However, it is often desireable to identify the presence of a specific type of acid gas, and to quantify the amount of the specific acid gas. The acid gas monitor in application Ser. No. 534,701 now U.S. Pat. No. 5,032,721 provides the requisite specificity to detect certain types of acid gas (such as HF), but it lacks the specificity for others. If more than one acid gas is present, the specificity is inadequate to distinguish between the gases. For example, if chlorine is being monitored and another acid gas is present, the device may give a false indication of the presence of chlorine or its concentration.